Volume reabsorption by the loop of Henle: a micropuncture study

Micropuncture analysis of single-nephron function in NHE3-deficient mice

The Na/H exchanger isoform 3 (NHE3) is expressed in the proximal tubule and thick ascending limb and contributes to the reabsorption of fluid and electrolytes in these segments. The contribution of NHE3 to fluid reabsorption was assessed by micropuncture in homozygous (Nhe3-/-) and heterozygous (Nhe3+/-) knockout mice, and in their wild-type (WT, Nhe3+/+) littermates. Arterial pressure was lower in the Nhe3-/- mice (89 +/- 6 mmHg) compared with Nhe3+/+ (118 +/- 4) and Nhe3+/- (108 +/- 5). Collections from proximal and distal tubules demonstrated that proximal fluid reabsorption was blunted in both Nhe3+/- and Nhe3-/- mice (WT, 4. 2 +/- 0.3; Nhe3+/-, 3.4 +/- 0.2; and Nhe3-/-, 2.6 +/- 0.3 nl/min; P < 0.05). However, distal delivery of fluid was not different among the three groups of mice (WT, 3.3 +/- 0.4 nl/min; Nhe3+/-, 3.3 +/- 0.2 nl/min; and Nhe3-/-, 3.0 +/- 0.4 nl/min; P < 0.05). In Nhe3-/- mice, this compensation was largely attributable to decreased single-nephron glomerular filtration rate (SNGFR): 10.7 +/- 0.9 nl/min in the Nhe3+/+ vs. 6.6 +/- 0.8 nl/min in the Nhe3-/-, measured distally. Proximal-distal SNGFR differences in Nhe3-/- mice indicated that much of the decrease in SNGFR was due to activation of tubuloglomerular feedback (TGF), and measurements of stop-flow pressure confirmed that TGF is intact in Nhe3-/- animals. In contrast to Nhe3-/- mice, normalization of early distal flow rate in Nhe3+/- mice was not related to decreased SNGFR (9.9 +/- 0.7 nl/min), but rather, to increased fluid reabsorption in the loop segment (Nhe3+/+, 2.6 +/- 0.2; Nhe3+/-, 3.6 +/- 0.5 nl/min). We conclude that NHE3 is a major Na/H exchanger isoform mediating Na+ and fluid reabsorption in the proximal tubule. In animals with NHE3 deficiency, normalization of fluid delivery to the distal tubule is achieved through alterations in filtration rate and/or downstream transport processes.

Nephron responses to converting enzyme inhibition in non-clipped kidney of Goldblatt hypertensive rat at normotensive pressures

To evaluate the effects of angiotensin converting enzyme inhibition (SQ 20881, CEI) on superficial nephron function of the non-clipped kidney in Goldblatt hypertensive rats in the absence of alterations in renal arterial pressure, control renal arterial pressure (RAP) was reduced first to the range generally obtained during CEI (124 +/- 4 mm Hg). RAP was maintained during the CEI period by adjustment of a suprarenal aortic clamp. At the reduced RAP, whole kidney and single nephron glomerular filtration rates (GFR) were reduced from the hypertensive levels and were lower than the measurements in normotensive control rats. During CEI, whole kidney GFR and single nephron GFR increased by 55 and 42%, respectively. There were decreases in absolute as well as fractional proximal reabsorption rates. In the intermediate nephron segment, fractional reabsorption was decreased, but absolute fluid reabsorption increased in proportion to the increased delivery rate. Proximal tubule and peritubular capillary hydrostatic pressures increased significantly during CEI also. These results indicate that an increased activity of the renin-angiotensin system occurring in Goldblatt hypertensive rats subjected to aortic constriction exerts effects to lower GFR and increase proximal reabsorption rate. The concomitant superficial nephron and whole kidney GFR responses to CEI when arterial pressure was maintained suggests that the pre-existing levels of angiotensin exerted similar influences on the total nephron population.